1. Field
The present technology relates to mechanisms for fastening and releasing a bit with respect to a mandrel.
2. Description of the Related Art
Screwdrivers have removable bits for engaging and driving screws into a workpiece. These screwdrivers typically have an elongate mandrel having an end into which a screw bit is removably coupled. Screws have a wide variety of screw drives in the head of the screw, including for example slotted, Phillips head, Robertson (square) and hex to name but a few. Each time a different type of screw drive is used, the screw bit of the screwdriver needs to be changed out.
In many screwdrivers, the bit is coupled to the mandrel by threads. For example, in a power screwdriver disclosed in U.S. Pat. No. 4,146,071 to Mueller et al., issued Mar. 27, 1970, the bit has a reduced diameter externally threaded male portion to be received within an internally threaded female socket in the mandrel. Threaded couplings have the disadvantages that the mandrel and bits are both expensive and it is also difficult and time consuming to change the bit.
The power screwdriver of U.S. Pat. No. 4,146,071 utilizes a system in which the head of a screw is located and retained in coaxial alignment with the mandrel and bit by the head of the screw engaging a part-cylindrical guideway member having a diameter approximately equal to the diameter of the head of the screw. In such a configuration, it is necessary that the mandrel and bit be of a sufficiently small diameter that the mandrel and bit may reciprocate axially through the part-cylindrical guideway member. The constraints of the mandrel and bit being of a diameter not greater than the diameter of the screw head renders replacement of the threaded coupling of the bit to the mandrel with another system difficult.
Other bit to mandrel coupling systems are known in which the mandrel carries a split-ring in a deep groove in a socket in the mandrel. A split-ring is a ring of resilient material, typically metal, where the ring is split so that it can expand around larger diameter objects and contract back to its original size when in an unbiased position. One such example is shown in prior art FIG. 1, which shows a mandrel 20 including a split-ring 22 partially engaged in a mandrel groove 24 (all of which are shown in cross-section). FIG. 1 further shows a bit 26 engaged within the mandrel 24 and driving a screw 30 into a workpiece 32.
The bit 26 for use with mandrel 20 also includes an annular groove 28 around an outer circumference of the bit 26. When the bit 26 is inserted into the mandrel 20, the split-ring 22 retains the bit 26 in the mandrel 20 by the split-ring 22 being partially received in the groove 24 in the mandrel and partially within the groove 28 around about the bit. Such known systems suffer the disadvantage that the coupling may generally hold the bit too loosely so that it may come off the mandrel too easily, or hold the bit too tightly so that it is difficult to remove. Furthermore, as parts wear over time, the force required to remove the bit may decrease. Thus, even if the correct balance (not too loose, not too tight) is provided initially, over time, this balance may shift to a situation where the bit is held too loosely.